SPC 1300Z Day 1 Outlook

Day 1 Outlook Image

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0645 AM CST Mon Nov 30 2020

Valid 301300Z - 011200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM EASTERN NORTH
CAROLINA TO NEW JERSEY...

...SUMMARY...
At least isolated damaging winds and a couple of tornadoes will be
possible today from parts of Florida into the Carolinas,
Mid-Atlantic and southern New England.

...Synopsis...
A progressive, rather highly amplified synoptic pattern will
characterize the mid/upper troposphere over the CONUS.  The most
important feature will be a strengthening trough -- now apparent in
moisture-channel imagery from Lake Superior across MO to east TX.  A
large, closed cyclone should develop along the trough, over the
east-central CONUS, between 00-06Z.  Between 00-12Z, the
corresponding 500-mb low should move northeastward up the
Appalachians from eastern TN to western PA.  A shortwave trough --
initially over the southern Appalachians -- will accelerate
northeastward and weaken today, reaching western NY by 00Z.

The 11Z surface analysis showed the associated synoptic cyclone over
the western NC/southwestern VA/northeastern TN area, with a couple
embedded lows.  One was a frontal-wave low in the CLT/HKY area that
should become the primary cyclone center with time as it ripples
northeastward.  A cold front was drawn from that low across central
SC, southeastern GA, northwestern FL, and the eastern through
south-central Gulf.  The low should reach central NY by 00Z, then
decelerate and move erratically near Lake Ontario or northern NY
through tonight as it occludes more deeply.  At that time, the cold
front should reach the coastal Mid-Atlantic, then offshore of the
Carolinas to southeastern FL.  The warm front will move northward
through the Mid-Atlantic to southern New England today.

...Mid-Atlantic to southern New England...
Scattered thunderstorms are expected to continue on both sides of
the warm front, within a large mass of convection and precip,
becoming isolated to widely scattered southwestward across the warm
sector to the cold front.  As this regime shifts northeastward
through the Carolinas and Mid-Atlantic to southern New England today
through this evening, sporadic damaging to severe gusts and a few
tornadoes are possible.

A combination of subtle diurnal heating (beneath abundant cloud
cover) and boundary-layer theta-e advection will contribute to
enough destabilization to offset poor midlevel lapse rates, yielding
a northeastward-shifting corridor of 500-1000 J/kg MLCAPE over the
"slight" area.  This buoyancy will diminish northeastward into
southern New England along and south of the warm front where theta-e
will be weaker.  A shallow stable layer may persist near the surface
in northern fringes of the "marginal" area, but with potential for
momentum transfer in the strongest convective downdrafts to
penetrate that at strong-severe levels.

As the ejecting shortwave passes by to the west, the low-level
cyclone deepens, and the synoptic trough approaches, deep-layer flow
will strengthen region-wide.  45-60-kt effective-shear vectors
should develop over the warm sector, where a 40-65-kt LLJ also
should develop.  The later will lead to the potential for somewhat
curved and lengthy hodographs supporting supercells and small bows,
in a low-LCL setting, supporting some tornado potential in addition
to strong-severe gusts.

...FL...
Widely scattered to scattered thunderstorms are evident in a
pre-frontal convective band from northeastern FL to near SRQ. 
Though isolated convection is possible to its southeast, this
primary convergence line should be the main convective focus through
this afternoon as it crosses the peninsula and reaches at least the
lower Keys.  Isolated damaging to severe gusts are possible.

The front will encounter a progressively more-unstable boundary
layer in central/southwestern FL, and existing rich moisture from
there toward the lower Keys, related to a reservoir of maximized
theta-e now analyzed to their west.  Preconvective veering of
surface winds is forecast, which will be involved with some
counterbalancing effects:
1.  Reducing low-level and deep/speed shear, though the latter still
may be sufficient to support isolated organized convection, with
strong (50-60-kt) 500-mb flow over the surface boundary.
2.  Reducing of kinematic contribution to low-level
convergence/lift, though diurnal surface destabilization -- and
related weakening of MLCINH -- will help to maintain some
convective-scale lift as convergence gradually lessens.
3.  Increase in boundary-layer moisture as the increasing westerly
component advects Gulf parcels into the region and erodes an axis of
antecedent min theta-e.

..Edwards/Broyles.. 11/30/2020

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